1. Field of the Invention
The present invention generally relates to mobile communications, and especially relates to an apparatus and a method of determining weights of two or more transmitting antenna elements of a mobile terminal. Further, the present invention also relates to an apparatus and a method of measuring transmission power transmitted by a radio terminal.
2. Description of the Related Art
When designing mobile terminals, a representative example being portable telephones, one important concern is to raise transmission efficiency, given that a part of the transmission power is lost to (attenuated by) objects such as a human body in the neighborhood of the mobile terminal, and the transmission efficiency is degraded. Any objects, not limited to the human body, that intercept radio wave propagation can cause degradation of transmission efficiency.
It is generally known that the transmission efficiency can be improved by feeding power simultaneously to two or more antenna elements, wherein amplitude and a phase are appropriately adjusted such that antenna directivity is obtained. Differences in the amplitude and the phase between the antenna elements are referred to as “weight”, “feeding weight”, and the like. Technologies in this regard are disclosed by Non-patent References 1 and 2, for example.
According to the conventional weight determination method, initial weight (a relative gain and phase difference) is first given to two or more antenna elements of a portable telephone. Next, a radio wave is transmitted using all the antenna elements and the radiant efficiency is calculated and/or measured. Furthermore, values of the weights given to the antenna elements are changed, a radio wave is transmitted by all the antenna elements, and radiant efficiency is calculated and/or measured. Subsequently, radiant efficiency is obtained for all the weights (i.e., all combinations of gains and phase differences). Then, a weight that gives the best radiant efficiency is made the weight of the transmitting antenna element of the mobile terminal.
Further, a radiation-pattern integration method is known as another technique of measuring radiant efficiency, and measuring radiation power that serves as the base of the radiant efficiency. The radiation-pattern integration method obtains an antenna radiation pattern by a fixed sensor measuring the power from the radio terminal, wherein the power is measured and integrated over the whole region (all solid angles, including all vertical and horizontal angles) that surrounds the radio terminal, while relative positions between the radio terminal (such as a mobile terminal) and the sensor are changed. The sensor may receive horizontal and vertical polarization components simultaneously with two polarization antennas that intersect orthogonally, or alternatively, may switch the arrangement directions of one polarization antenna such that each component is separately received. The radiation-pattern integration method is disclosed by Non-patent References 3 and 4, for example. Another method called random field method is also practiced, which is a power measuring method that uses a great number of objects (scattering objects) that are appropriately arranged in a room where measurement is performed such that a radio wave transmitted by the radio terminal is scattered, and the radiant efficiency is obtained from statistics of the received waves that are scattered by the scattering objects.
[Non-patent Reference 1] Tomoaki Nishikido, et al., “Two parallel element distributed feeding antenna element for portable radio terminals”, Shin-gaku Society Convention B-1-190, 2003
[Non-patent Reference 2] Ryo Yamaguchi, et al., “Improvement in efficiency of the cellular phone by distributed feeding”, Shin-gaku Society convention B-4-4, 2003
[Non-patent Reference 3] Sakuma, et al., “A study of radiant efficiency measurement of a small antenna”, Electricity-related Academic Conference Hokkaido Branch Union Convention, Fiscal year Heisei 12, p. 202
[Non-patent Reference 4] Qiang CHEN, et al., “Measurement of Power Absorption by Human Model in the Vicinity of Antennas”, IEICE TRANS. COMMUN. VOL. E80-B, NO. 5, MAY 1997, pp. 709-711